This study systematically investigates the low-cycle fatigue (LCF) behavior of [001]-oriented René N5 single-crystal superalloy under variable-amplitude loading at 760 °C, with a focus on nonlinear damage accumulation mechanisms. Through coordinated experimental characterization and model development, constant-amplitude tests established baseline strain-life relationships (4740 cycles at Δε/2 = 0.7%, 1713 cycles at Δε/2 = 0.8%), revealing cyclic hardening–softening transitions coupled with microstructural evolution. A novel damage accumulation model was proposed, integrating static toughness degradation dynamics and load-sequence interaction coefficients to address path-dependent nonlinearities. Comparative validation demonstrated the model's superiority over Miner's rule, particularly under high-to-low loading sequences. These findings provide critical insights for remaining life assessment of turbine components subjected to complex loading.